The invention is directed to depositing a thin film on a substrate. More particularly, the preferred embodiment relates to controlling the deposition of thin films on a substrate during film deposition and controlling outboard shadowing and thus inboard-outboard asymmetry.
A variety of deposition techniques are known for depositing thin film material. Such techniques include sputter deposition, ion beam sputter deposition (IBD), and long-throw physical vapor deposition (PVD) systems. PVD is a thin film deposition process in the gas phase in which source material is physically transferred in a vacuum to a substrate without any chemical reaction involved. PVD includes both thermal and e-beam evaporation and sputtering. Additionally, thin films can be deposited using low pressure chemical vapor deposition in which chemical vapor deposition is performed at a pressure below atmospheric pressure.
Many of these deposition processes require deposition of thin films on substrates having particular topographical features that affect the distribution and properties of deposited material across the substrate. For example, lift off deposition processes are used in many important thin film feature fabrication processes, such as in the manufacture of magnetic heads and semiconductor devices. An exemplary substrate 10, i.e., a wafer, showing layout features 12 thereon is illustrated in FIG. 1. Notably, layout features 12 are typically fabricated from photoresist, which is selectively removed according to the written pattern after a lift off step. Lift off deposition processes allow definition of a pattern on a wafer surface without etching, and are typically used to define geometry of hard to etch metals, such as gold. In such processes, metal is lifted off in selected areas by dissolving underlying resist.
In a typical IBD process, for example, the substrate 10 is rotated during am deposition about a central axis or center 44 (FIG. 3). Features 12 on the substrate 10 have an inboard side 22, which is the side facing toward the center 44, and an outboard side 24, which is the side facing away from the center 44, these sides being illustrated in FIGS. 2A, 2B, and 2C. As discussed in further detail below, control of the deposition profile on the inboard/outboard sides of a feature is often critical to device performance.
IBD is particularly well suited for lift off processes due to some unique features IBD possesses. The low process pressures and directional deposition are chief among them. These enable the lift-off step to be extremely clean and repeatable down to very small critical dimensions, e.g., for example, less than 0.5 xcexcm.
In recent years, IBD has become the method of choice for deposition of stabilization layers for thin film magnetic heads because such an application requires a lift off step subsequent to the deposition of the stabilizing material.
In addition to good lift-off properties, IBD films have extremely good magnetic properties. Additionally, in IBD processes it can be very convenient to position system components to optimize the properties of the deposited film and to rotate the substrate to average out certain non-uniformities introduced by the tilting and other process steps.
For most applications, control of the deposited material onto the substrate is needed. In the fabrication of structures in which one axis is much longer than the other, e.g., in optical cross connect micro-electro mechanical systems (MEMS) where there is a very long vertical flap inside a wide trench, deposition control is critical. In particular, without sufficient control of the deposited optically reflecting metal coating, the flap can buckle due to the stress imbalance on the opposite sides of the flap. More generally, various standards relating to material deposition have been developed for the fabrication of semiconductor devices.
Next, variations in the thickness of the thin layer is a common problem in thin film deposition. As known in the art, these variations are exacerbated when, for example, photoresist masks are used in the lift-off steps. Techniques have been developed to control the overall thickness of layers of deposited materials onto the substrate. For example, a flux regulator has been used to help control the overall thickness of deposited thin layers by impeding the path of portions of the sputtered beam.
However, flux regulators have not been used to address problems associated with asymmetry in sidewall profiles. It is desirable to have symmetric profiles of the deposited material across the sidewall of device features on a substrate because otherwise device performance can be severely compromised. For example, in the manufacture of magnetic heads, the symmetry of the profile of the deposited material obtained after the lift-off step is imperative for stable performance of the device. Therefore, ideally, the deposition is controlled to maintain an appropriate profile.
A drawback of previous thin film deposition processes is that they cannot adequately control the profiles on either side of the photoresist, even when known flux regulators are used. One cause of this is the so-called xe2x80x9cinboard-outboardxe2x80x9d effect. This means that one side of a feature is more heavily coated than the other side, thus creating an asymmetric profile. This effect is a result of the fact that an off-center point on the substrate is bombarded by more atoms incident from the inboard side of the feature than the outboard side, for example, when the center axes of the target and substrate are collinear. This asymmetry is usually most pronounced at the edge of the substrate.
The source of this problem is related to the divergence of the deposition flux. Based on the geometry of the set-up, this divergence causes variations in the beam that impinges upon the substrate. As a result, asymmetric shadowing of the features occurs and creates an asymmetric profile of the deposited material, as shown is in the prior art depictions in FIGS. 2A-2C. FIG. 2A shows asymmetric deposition 20 and 20xe2x80x2 on an inboard side 22 and an outboard side 24, respectively, of a lift-off photoresist feature 12 on a substrate 10. In this case, the slope of the profile at the inboard side 22 is significantly steeper than the slope of the profile at the outboard side 24, which can substantially compromise device performance. Again, ideally, these sidewalls are not sloped, i.e., the sidewalls are perfectly vertical for optimum device performance.
FIGS. 2B and 2C show basic elements that represent actual device features that may be more complicated, e.g., with multiple layers, more complicated topography. The step feature 112 of FIG. 2B represents, for example, the contact formed by the leads and the permanent magnet layers on the walls of the MR sensor shown in FIG. 9 of U.S. Pat. No. 6,139,906 to Hedge et al., the entirety of which is incorporated by reference herein. This is actually just as critical for the device performance as the slope of the deposited film formed with the lift-off mask that is discussed above. Alternatively, the step feature 112 represents the long vertical flap of an optical cross connect MEMS device, in which case without sufficient control of the deposited optically reflecting metal coating. Alternatively, the device may be a laser bar or integrated laser device on a wafer, in which case the sidewalls of feature 112 would reflect the laser facets, and the coating would be a reflective or antireflective coating. Control of such coating thicknesses are critical to the laser performance.
FIG. 2B depicts asymmetric deposition 120 and 120xe2x80x2 on an inboard side 122 and an outboard side 124, respectively, of a permanent photoresist feature 112 on a substrate 110. In the prior art, typically, the inboard side 122 of the step feature 112 has more material deposited thereon.
The trench feature 212 of FIG. 2C is one that is commonly found in microelectronic device manufacturing. When a certain material is etched with a photoresist mask, the mask is removed, and a second layer of material is deposited over the trench feature 212 in a continuation of the device process. Trench feature 212 may be the result of a series of successive coatings where the patterning process occurred below the surface of the feature 212 shown, but where each subsequent coating, by conforming to the patterned feature 212, transferred the trench feature 212 to the next coating step, respectively. This could be for example an ultrathin corrosion resistant coating deposited on a completely patterned thin film magnetic head transducer, which must be as thin and conformal to the patterned surface as possible.
FIG. 2C illustrates asymmetric deposition 220 and 220xe2x80x2 on an inboard side 222 and an outboard side 224, respectively, of a trench feature 212 on a substrate 210. In the prior art, the inboard side 222 of the trench feature 212 typically was deposited with more material than the outboard side 224, resulting in an asymmetric deposition, as is shown in FIG. 2C. Again, such an asymmetric deposition profile can significantly compromise device performance.
In sum, the art of thin film deposition was in need of a method and apparatus of controlling deposition profiles, and particularly, inboard/outboard asymmetry relative to device features.
The preferred embodiment overcomes the drawbacks associated with prior systems by providing a deposition system that minimizes the occurrence of asymmetric deposition profiles. The invention achieves symmetric profiles, in the first instance, by tilting the substrate to provide non-normal flux incidence on the substrate. Furthermore, the preferred embodiment utilizes one or more strategically shaped and positioned profiler masks that selectively block portions of the flux to specifically obviate the problem of deposition asymmetry. In an alternative, the profiler mask(s) is modified to also provide uniformity shaping of the deposited material.
According to one aspect of the preferred embodiment, a deposition system includes a substrate holder supporting a substrate defining at least one topographical feature. In addition, the system includes a deposition flux that is directed toward the substrate. A first profiler mask is positioned between the deposition flux and the substrate, and is shaped so as to reduce inboard/outboard asymmetry in a deposition profile associated with the feature.
In accordance with another aspect of this preferred embodiment, the profiler mask has a shape of a sector of a circle. In accordance with yet another aspect of this preferred embodiment, the profiler mask includes a solid portion and an open portion.
According to another aspect of the preferred embodiment, an apparatus to reduce inboard/outboard asymmetry in thin-film profiles includes a profiler mask that is disposed between a deposition flux directed toward a substrate and the substrate. The profiler mask being shaped so as to block the same amount of arc of the substrate along a length of the mask.
According to yet another aspect of the preferred embodiment, a method of controlling deposition asymmetry on sides of features disposed on a substrate includes directing a deposition flux toward the substrate. The substrate is tilted so that the deposition flux impinges on the substrate at a non-normal incident angle. A first profiler mask is inserted between the deposition flux and the substrate to at least partially block the deposition flux so as to reduce inboard/outboard asymmetry.
In accordance with another aspect of this preferred embodiment, the profiler mask is inserted in a region at about a position furthest away from the sputter target. In accordance with yet another aspect of this preferred embodiment, the deposition flux is generated by directing a beam of ions toward a target of a material to be sputtered. A second profiler mask is inserted between the deposition flux and the substrate to at least partially block deposition of the thin film on the substrate. In a preferred embodiment, first and second profiler masks are inserted 90 degrees and xe2x88x9290 degrees, respectively, relative to either side of a point furthest away from the sputter target.